Internal stresses and the cyclic deformation of an aluminium matrix composite

Thesis/Dissertation · Wed Jan 01 04:00:00 EST 1992

OSTI ID:7236241

Johannesson, B

The development of internal stresses in planar random Saffil fibre reinforced aluminum with a range of fibre volume fraction has been studied theoretically and in monotonic and cyclic deformation (Bauschinger) experiments at room temperature and 77K. The Eshelby S tensor for a planar random array of fibres is calculated. Because the aluminum/Saffil composites are not isotropic in the transverse directions, the plastic strain used in the calculations has to be determined experimentally. A method for quantifying the anisotropic plastic flow of aluminium/Saffil composites is proposed and the results are used in calculations of the mean stress hardening rate. The measurements of the thermal stresses obtained in the Bauschinger experiments are in quantitative agreement with results obtained in monotomic tests. The magnitude of the thermal stress at room temperature or 77K is independent of fibre volume fraction and a comparison with predictions shows that relaxation of the thermal stress increase with fibre volume fraction. Cycling in the Bauschinger experiments reduces the thermal stress and hence the separation of the mean and thermal stresses is essential or a reliable measurement of the mean stress hardening rate. Matrix hardening contributes considerably to the overall hardening of the composite, both at room temperature and 77K. The modified Orowan-Wilson model, which enables the plastic friction coefficient to be measured in copper-tungsten composites, has been applied to the aluminium/Saffil composites. The model requires both the mean stress and the peak stress curve obtained in Bauschinger experiments to be linear in plastic strain. The temperature dependence of the mean stress hardening rate suggests that relaxation is thermally activated. A model for relaxation of the mean stress is proposed. An equation is derived for the number of Orowan loops per fibre and it is assumed that the rate controlling mechanism of relaxation is cross slip of screw dislocations.

• Research Organization: Surrey Univ., Guildford (United Kingdom)
• OSTI ID: 7236241
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
360603* -- Materials-- Properties
COMPOSITE MATERIALS
DEFORMATION
MATERIALS
MATRIX MATERIALS
RELAXATION
STRESS ANALYSIS
STRESS RELAXATION